The present invention relates to satellite based mobile communications systems, and more particularly, to the utilization of polarization diversity to reduce fading in such systems.
Digitally modulated satellite and terrestrial mobile communications systems generally receive only a single polarization of a transmitted signal (e.g. vertical, horizontal, left-hand or right-hand circular). However, electromagnetic waves propagating in the vicinity of a mobile communications system user are subjected to significant scattering and because the sources of scattering are randomly oriented, there is cross coupling of polarizations. The embodiments of this invention take advantage of the uncorrelated nature of each polarization path to reduce the depth of fades by receiving and combining dual polarizations.
It is highly desirable to be able to use mobile satellite communication devices inside of buildings and automobiles. Unfortunately, maintaining a reliable link in these environments generally requires significant raw link margin. The additional margin is principally obtained by increasing radiated power and antenna size. Both of these options are very costly. Increasing radiated power increases power consumption and decreases operating time on a single battery charge. Increasing the satellite antenna gain by increasing antenna aperture size causes difficulties in terms of power consumption, physical size, stowage, and deployment.
Difficult propagation environments such as in-building communications contain significant randomly oriented scatterers and reflectors which tend to randomize the polarization of the signal at the receiving antenna. Additionally, in these multipath environments, signal fades in one polarization tend to occur at different times than signal fades in an orthogonal polarization. Therefore, there is a need for a satellite mobile communications system that can take advantage of the power available in the cross-polarized signal.